The present invention pertains to methods and apparatus for radioactive well logging. More particularly, the present invention relates to techniques for determining azimuth and speed of horizontal fluid flow in earth formations in the vicinity of a well borehole by the employment of neutron activation of the fluid and the analysis of resultant gamma radiation based on count rate data obtained as functions of horizontal direction and time.
In secondary and tertiary recovery of petroleum deposits, many of the recovery techniques employ the injection of water or chemical solutions into the earth formations comprising the reservoir from injection wells. In planning the recovery operation, the injection of water or chemicals has, in the past, been limited by certain assumptions and/or approximations concerning the mobility of fluid in the formation comprising the reservoir. Crucial factors in such fluid injection programs include the vertical conformity of the producing formation as well as the horizontal permeability and uniformity. In some reservoirs, formation lensing or horizontal partitioning by permeability barriers, such as faults, can occur. In such instances, apparently correlevant intervals of permeability may be separated from one well to another in the field by such formation lensing or permeability barriers being interposed across the interval of formation between the wells.
It is therefore apparent that some knowledge of the existence of such lensing or permeability barriers could prevent the injection of large amounts of costly chemicals or water into a recovery pattern in which formation continuity between injection wells of a field is partially or totally absent. This could prevent the expenditure of large sums of money, time and effort and a fruitless project to recover secondary or tertiary problem deposits.
Information concerning such structure of the earth formation may be obtained by an evaluation of the direction and speed of formation fluid flow by a borehole in the field. Also, by obtaining such speed and direction of flow measurements at a sufficient number of boreholes throughout the field, a mapping of the total flow throughout a petroleum reservoir may be obtained to help in the operational planning of injection of chemicals or water to assist in determining optimum withdrawal rates. Moreover, a knowledge of the lateral water flow characteristics of a particular formation in a producing field can help greatly in understanding of the reservoir dynamics of the particular reservoir being produced.
It is sometimes desirable in a reservoir with multiple producing intervals for a reservoir engineer to be able to delineate those producing zones which provide the most water influx or water drive to the production of petroleum. The mapping of lateral water movement in all zones both above and below the expected water table in the producing formation should supply this information.
Reservoir engineers have been provided with relatively few and often inaccurate well logging instruments to determine the vertical conformance characteristics of the earth formations comprising a reservoir. As a result, they have suffered from inadequate information concerning the properties of the earth formations making up the reservoir. Radioactive tracer studies of the movement of fluids in the vicinity of a well borehole can be misleading in this respect because of the lack of uniform absorption of the tracer element into the flowing stream of formation water. Also, it is time consuming to provide tracer isotopes with sufficient half life to be injected in an injection well, and observe their movement days or even weeks later at a monitoring or producing well in order to obtain some idea of the lateral flow speed or velocity of fluids in the formations comprising the reservoir. In the present invention, relatively little time delay from the beginning of injection is required to observe fluid movement as this is substantially only a function of hydraulic pressure buildup in the formation.
U.S. patent application No. 698,394, filed June 21, 1976, discloses techniques for analyzing gamma ray count data obtained from activated formation fluid to reveal the horizontal flow speed of the fluid. Measurements of count rate intensity, taken as a function of time, are utilized for this purpose. Information concerning the direction of the horizontal formation fluid flow may be obtained as disclosed in U.S. patent application No. 815,964, filed July 15, 1977. A cluster of three gamma ray detectors is used to simultaneously obtain three count rates. The relative intensities of the count rates, compared to the geometry and orientation of the detector cluster, are interpreted to reveal the azimuthal angle of the flow of the formation fluid.